1. Field of the Invention
The present invention relates to a process for producing a particle-reinforced titanium alloy which is reinforced by ceramic particles having a thermodynamically stable property in titanium alloy.
2. Description of the Related Art
There has been known particle-reinforced titanium alloy which is reinforced by particles. As a technique for producing this type titanium alloy, Japanese Unexamined Patent Publication 10-1,760 has been provided. This Patent Publication technique includes: (1) using titanium alloy which is reinforced by dispersing ceramic particles having a thermodynamically stable property, such as titanium boride, in a matrix, and (2) heat-treating this titanium alloy to dissolve a colony grain structure and to generate a minute-acicular xcex1 phase structure. According to the process disclosed in this publication, the above-mentioned particle-reinforced titanium alloy is produced by way of the steps including: (1) heating the titanium alloy in a temperature range not less than xcex2-transus temperature; (2) quenching the titanium alloy with water from the temperatures range not less than xcex2-transus temperature to room temperature or to under room temperature; and (3) heating the titanium alloy in a two phase region of (xcex1+xcex2) formed between xcex2-transus temperature and 800xc2x0 C. The quenching step indicates a considerably rapid cooling rate.
Also, Japanese Unexamined Patent Publication 3-73,623 discloses another process for heat-treating a xcex1+xcex2 type titanium alloy. This process includes: (1) heating the titanium alloy having a xcex1+xcex2 type in a temperature range which is 10-60xc2x0 C. lower than xcex2-transus temperature; and (2) cooling the titanium alloy at a cooling rate of 0.1-5xc2x0 C./second to less than 500xc2x0 C. so as to improve toughness thereof. When heating temperature is not less than xcex2-transus temperature, a phase of xcex2 easily becomes a large-size. In this publication technique, it is guessed that heating temperature is set at temperatures which is 10-60xc2x0 C. lower than xcex2-transus temperature for avoiding a large-sized phase of xcex2.
The technique disclosed in Japanese Unexamined Patent Publication 10-1,760 intends to improve fatigue strength of titanium alloy; however, it does not intend to improve creep resistance. When the heat treatment disclosed in this publication is carried out, acicular xcex1 phases are parted and then change into broken-up structures; therefore, creep property is deteriorated in spite of high fatigue strength. Generally, it is thought that a finer microstructure leads to improved fatigue strength and that a larger microstructure leads to suppressed creep deflection and improved creep resistance.
Also, the technique disclosed in Japanese Unexamined Patent Publication 3-73,623 intends to improve toughness; however, it does not intend to improve creep resistance. Further, the titanium alloy disclosed in this publication does not contain particles such as titanium boride particles, and heating temperature does not exceed xcex2-transus temperature.
The present invention has been accomplished in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a process for producing particle-reinforced titanium alloy which is excellent in creep resistance while ensuring fatigue strength.
The present inventors have eagerly researched titanium alloy and have developed the present invention by experimentally confirming the following phenomenon. When the present inventors have carried out: using titanium alloy in which ceramic particles are dispersed having a thermodynamically stable property; heating the titanium alloy in a temperature range of not less than xcex2-transus temperature; and cooling the titanium alloy at a cooling rate of 0.1-30xc2x0 C./second: titanium alloy is improved in creep resistance while ensuring fatigue strength.
The reason for obtaining the above-mentioned characteristics is not surely clear. However, this reason is guessed as follows:
It is thought that a larger microstructure contributes to suppress creep deflection and to improve creep resistance, and that a finer microstructure contributes to improve fatigue strength. The present invention uses the titanium alloy in which ceramic particles having a thermodynamically stable property are dispersed. Therefore, the present invention prevents abnormal growth of the old of xcex2 grain, in spite of the complete acicular formation of microstructures, even when the titanium alloy is heated in a temperature range of not less than xcex2-transus temperature. Also, since the titanium alloy is cooled from the range of not less than xcex2-transus temperature, and since the titanium alloy passes through xcex2-transus temperature at an appropriate cooling rate of 0.1-30xc2x0 C./second, the microstructure size of titanium alloy is appropriate in such a manner that both creep resistance and fatigue strength are ensured.
The present invention provides a process for producing a particle-reinforced titanium alloy, which comprises the steps of: heating a titanium alloy in which ceramic particles having a thermodynamically stable property are dispersed in a temperature range of not less than xcex2-transus temperature; and cooling the heated titanium alloy to pass through the xcex2-transus temperature at a cooling rate of 0.1-30xc2x0 C./second.
The present invention can provide a particle-reinforced titanium alloy in which creep resistance is excellent while fatigue strength is ensured.
The present invention employs titanium alloy in which ceramic particles having thermodynamically stable property are dispersed.
The titanium alloy may be a sintered compact formed by sintering a green compact, a forged product formed by forging the sintered compact, a cast product, or a forged product formed by forging the cast product. As for forging, hot forging can be used.
The titanium alloy can include an xcex1 phase-stabilizing element, for example aluminum (Al), and a xcex2 phase-stabilizing element. The titanium alloy can contain, at least, 3-6% of aluminum (Al), and 2-6% of tin (Sn) by weight, when a matrix of titanium alloy is 100% by weight. However, the present invention process is not limited within these contents.
The microstructure of matrix of the titanium alloy in an ordinary-temperature region may be a microstructure wholly formed of xcex1 phases, a microstructure mainly formed of xcex1 phases, or a microstructure formed of xcex1 phases being mixed with xcex2 phases. The xcex1 phase may be an acicular xcex1 phase, or an acicular xcex1 phase mixed with an equi-axed xcex1 phase.
The ceramic particles having a thermodynamically stable property may be titanium boride (TiB and TiB2), titanium carbide (TiC and TiC2), titanium silicide, and titanium nitride (TiN). In particular, titanium boride is preferable in such ceramic particles. Titanium boride can work as a hard particle or a reinforcing particle in a matrix of titanium alloy. Titanium boride has good congeniality for the matrix of titanium alloy; so, it is suppressed that a weak reactive phase for causing fatigue crack is formed in an interface between the titanium boride and the matrix of the titanium alloy.
Proportion of the ceramic particles having a thermodynamically stable property, such as titanium boride, can be chosen depending on applications, etc. An upper limit of the proportion may be 10% or 7% by volume, and a lower limit may be 0.1% or 0.4% by volume, in the case where the whole titanium alloy with ceramic particles dispersed therein is 100% by volume. However, the proportion of the ceramic particle is not limited within these ranges.
An average particle size of ceramic particles having a thermodynamically stable property, such as titanium boride, can be chosen depending on applications, etc. For example, an upper limit of the average particle size of the ceramic particle may be 50 xcexcm. A lower limit of the average particle size of the ceramic particle may be 0.5 xcexcm. However, the average particle size of the ceramic particle is not limited within this range.
The present invention includes the step of heating the titanium alloy in which ceramic particles having a thermodynamically stable property (e.g. titanium boride) are dispersed in a temperature range of not less than xcex2-transus temperature. The old xcex2 grain is produced by such step. Means of this heating step may be induction heating, furnace heating, or other heating modes. Heating time can be chosen depending on heating conditions of induction heating or furnace heating, size of titanium alloy, etc. Since the ceramic particles having a thermodynamically stable property are dispersed in the titanium alloy, the present invention can prevent the size of the old xcex2 grain from excessively increasing, even when the size of the old xcex2 grain is to be excessively increased because of a long heating time.
The present invention includes the step of cooling the titanium alloy, in which the ceramic particles having a thermodynamically stable property are dispersed, from a temperature range of not less than xcex2-transus temperature at a cooling rate of 0.1-30xc2x0 C./second. Therefore, the titanium alloy is cooled to pass through xcex2-transus temperature at a cooling rate of 0.1-30xc2x0 C./second. The cooling rate of 0.1-30xc2x0 C./second is obtained generally by gas cooling, and it is considerably slower than that of quenching. A representative cooling mode may be a gas cooling mode utilizing rare gas as cold gas, and an air cooling mode.
According to the present invention, there can be obtained an appropriate matrix, and an appropriate size of the microstructure of titanium alloy with the ceramic particles such as titanium boride having a thermodynamically stable property dispersed therein.
A preferable mode of the present invention further includes the step of compressing the titanium alloy before such heating step. The compressing step is, for example, a step of forging the titanium alloy. In the compressing step, the titanium alloy, in which the ceramic particles having a thermodynamically stable property such as titanium boride are dispersed, is compressed in a two phase temperature range of xcex1+xcex2 or in a temperature range of not less than xcex2-transus temperature.
That is to say, the heating step is carried out after the titanium alloy is compressedxe2x80x94for example, by forging. The compressing step is carried out in the case where the matrix of titanium alloy is formed of a mixed phase of xcex1+xcex2 or a phase of xcex2. Density of the titanium alloy can be made advantageously higher by compressing the titanium alloy. Therefore, pores can be advantageously reduced in the case where the titanium alloy is formed by powder metallurgy.
The present invention includes the step of cooling the titanium alloy from the temperature range of not less than xcex2-transus temperature at a cooling rate of 0.1-30xc2x0 C./second. As mentioned above, this cooling rate is much slower than that of quenching. The cooling rate of 0.1-30xc2x0 C./second can improve creep resistance. Therefore, the present invention is suitable in producing high strength parts to be used in high-temperature atmosphere, such as valves of internal combustion engines, etc.
Moreover, it is preferable that titanium alloy has an elongation value over the predetermined value so as to ensure impact resistance of the titanium alloy. When the cooling rate is less than 0.1xc2x0 C./second, the elongation value is small, as shown in FIG. 2, and impact resistance is disadvantageous. The above-mentioned cooling rate is preferable in ensuring elongation and impact resistance. Accordingly, the present invention is suitable in producing high temperature strength parts formed of the titanium alloy, such as valves of internal combustion engines.
The induction heating can be used in heating the above-mentioned titanium alloy in a temperature range of not less than xcex2-transus temperature. In particular, high frequency induction heating is preferable. The induction heating can shorten heating time of titanium alloy and can improve cycle time of productivity. In addition, the induction heating effectively reduces an exposing time in which the titanium alloy is exposed to a high-temperature atmosphere to suppress surface oxidation of the titanium alloy and to advantageously reduce a machining margin of the titanium alloy.